[Fix]Update swinir_x2s48w8d6e180_8xb4-lr2e-4-500k_div2k.py, use MultiValLoop.

[ashutoshsingh0223]

ValLoop does not work well in mmagic and the training expects the validation to have certain methods/attributes only supported in MultiValLoop and not in ValLoop.

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[zengyh1900]

lgtm